Fields defined with an OCCURS clause are assigned unique sequential names. Fields without names are disregarded.
When multiple fields occupy the same area, the compiler chooses only one of them unless you have a WHEN directive to distinguish them.
To choose:
The compiler preserves fields mentioned in KEY IS phrases.
It discards group items unless USE GROUP is specified.
It discards REDEFINES.
It uses the largest record if there are multiple record definitions.